Sinkhole risk in Franklin County, IA: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 100%
At or near the surface: 0%
Buried under cover: 100%

The next step for one property

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 580 of Franklin County's 582 square miles on soluble rock. With about 10,024 residents (17 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 391 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why the 100% under cover still counts, and how these percentages are measured.

Counties around Franklin County

The nearest county rated sinkhole country is Clayton County, IA, about 99 miles east. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Cerro Gordo County, IA 23 mi north · 0% at surface
Wright County, IA 24 mi west · 0% at surface
Hardin County, IA 24 mi south · 0% at surface
Butler County, IA 25 mi east · 0% at surface
Hamilton County, IA 33 mi southwest · 0% at surface
Floyd County, IA 33 mi northeast · 0% at surface

Which formation it is

The largest mapped soluble unit here is the Kinderhook Series (limestone), Early Mississippian in age, covering about 78% of the county. Next is the Yellow Spring Group at 13%.

Against the rest of Iowa

Franklin County ranks 41 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.